MICROWAVE OVENS

Abstract: There is disclosed various microwave applicators which are positioned above an electrically conductive conveyor band on which products, such as biscuits in a baking oven, are carried to be subjected to microwave radiation, the applicator having a launch section configured to provide an electric field in the region occupied by the products above the band which is that which would exist in a parallel plate transmisssion line extending lengthwise of the oven, the electric field having a plane of polarity which is normal to the plane of the band. TEM and/or TM1 modes of the electric field are created. The applicator comprises symmetrically disposed feeder waveguides (3) extending across the conveyor bands, slots (8) in a longitudinally facing broad side (3') of the guides emitting waves into a first stage of a launch means (5), the first stage consisting of upper (6') and lower (6'') first stage plates, the upper plates (6') connecting with second stage plates (7) which themselves connect with further applicator panels (11). Main treatment spaces (7') and further treatment spaces (11') are defined.

Description: MICROWAVE OVENS BACKGROUND OF THE INVENTION This invention relates to microwave ovens particularly, but not exclusively, to industrial ovens for food products such as biscuits, snacks, chips, meat substitutes. The invention may however be applied to non-food products, such as to the heating of plastics. Continuous ovens for food products such as biscuits can be designed to have a plurality of zones through which each food product travels in sequence. Such zones can be used to provide different heating and baking conditions. Multi-media ovens combine different means of heating a food product in the different zones, such as cyclotherm radiant heating, gas or electrically heated convection, near infrared radiant heat, and are well-known in the industry. In addition to the usual methods of heating we have shown it to be advantageous to inject microwave energy into an oven at various positions along the oven length. Such an oven which employs microwave heating in addition to another form of heating is hereinafter referred to as a 'combined oven'. Microwave heating provides means to induce a rapid transfer of energy to the product, the level of energy transfer being selected to provide a desired effect within the product. In a proposed combined microwave-biscuit oven, using four microwave zones, the first microwave zone induces a rapid rise in temperature within the product, the second microwave zone enhances development, and the third and fourth microwave zones reduce the moisture content of the product prior to leaving the oven. Conventionally heated industrial ovens for food products generally use a metal band to support and convey the food products through the oven. The band may be a metal strip or a mesh band. The use of a metal band in a multi-mode microwave oven, however, gives rise to serious problems. The microwave heating efficiency is found to be reduced to unacceptably low levels, especially with thin food products such as biscuits. Industrial microwave units basically consist of a microwave generator and a microwave applicator. We have appreciated that one of the reasons for the low efficiency of prior art combined ovens is that the conventional applicators operate in several modes, and that the use of substantially one or two modes only can provide advantages. The terms 'band-parallel' and 'band-normal' used herein are intended to refer to directions which are parallel to and normal to, respectively, the plane of the oven band. Whilst the oven band will often be horizontal, it should be appreciated that the oven band need not always be horizontal since products can be conveyed on a sloping band. SUMMARY OF THE INVENTION We have previously proposed a microwave tunnel oven comprising a microwave applicator extending transversely of an electrically conductive oven band and intended to produce electric fields in the region above the band and adjacent thereto, with a plane of polarisation substantially perpendicular to said conductive oven band, and propagated in a direction substantially lengthwise of the band. The various aspects of the present invention are concerned with practical arrangements to achieve such electric fields. The band-parallel component (E^) of the electric field is constrained by the conductive band to be small in the region closely adjacent to the band. The vertical component (E . of the electric field in the region occupied by the product is arranged to be sufficient to provide heating of the product carrieα on the conductive band. The applicators in accordance with the present invention are essentially configured to create, in the absence of a product, radiation propagated lengthwise of the band in the (TEM) transverse electromagnetic mode, or TM-, mode. The presence of a product will distort the radiation pattern. Designs in accordance with the invention aimed at producing such an E-field take account of the conductive oven band in the overall design of the microwave applicator. If the head-space between the product and the applicator is small (such as 40 mm), the equivalent of a parallel-plate transmission line can be created, supporting the TEM mode. If the head space is greater than 60 mm, a higher order mode can be supported. In experiments in which microwaves are simply fed from the sides of the parallel-plate transmission line formed by the conductive band and a vertically spaced horizontal plate, and in the presence of the product, there was found to be a high attenuation of the microwave energy propagating between the band and overlying plate and normal to the axis of travel of the band. This caused severe non-uniformity of heating of the product on oven bands of width greater than 100 mm. We have designed applicator configurations which launch a quasi-plane wave longitudinally of the oven band, and thereby provide, in conjunction with the band, the equivalent of a parallel plate waveguide extending longitudinally of the oven. Such an applicator configuration can provide improved heating uniformity across the oven width. This requires a microwave applicator with substantially constant amplitude illumination in its aperture plane, and preferably utilising one or two modes only. Means for broadcasting such a waveform will be described hereinafter, and relate to a microwave generator frequency of 2450 MHz. Other (lower) frequencies can be used but, due to the resultant longer wavelength, less uniform illumination will be achieved. The microwave applicator preferably comprises an elongate feeder waveguide extending substantially transversely of the band and positioned above the band, the waveguide being provided with a plurality of longitudinally spaced-apart radiation emitter means adapted to emit radiation with a substantially vertically polarised electrical component in the near vicinity of the band, and preferably substantially throughout the vertical thickness of the product. A phased array of said emitter means is preferably employed to broadcast the waveform, that is there 1s a uniform phase difference between adjacent pairs of emitter means of the plurality of emitter means. The feeder waveguide is preferably a rectangular section waveguide. The emitter means may be slots in one wall of the waveguide or antenna associated with respective probes which extend inwardly of the waveguide from a wall of the waveguide. The emitter means are preferably spaced-apart along the waveguide on a pitch of substantially half a guide wavelength. When the emitter means are antennas the antennas may be carried on the underside of the. waveguide. Alternatively the antennas could be connected to respective probes located in the waveguide by respective waveguide links, such as coaxial waveguide links. When the emitter means are slots, each slot is preferably of a length substantially half a free-space wavelength. Such slots are preferably provided in a broad side of the waveguide which is oriented substantially normal to the band. According to one aspect of the invention we provide a microwave tunnel oven for subjecting products conveyed through the oven to microwave radiation comprising a tunnel oven casing, the oven casing comprising spaced apart tunnel oven side-walls and an oven roof connecting the side-walls, an electrically conductive product-supporting conveyor band positioned between the side-walls and beneath the roof to extend lengthwise of the tunnel, band drive means operative to drive the band, a microwave generator means, a microwave applicator positioned between the side-walls and above the band, microwave supply means connecting the generator means to the applicator, the applicator comprising an elongate microwave emitter assembly extending transversely of the oven band, microwave launch means extending longitudinally of the oven and above the band from adjacent to the emitter assembly, the emitter assembly comprising a feeder waveguide extending transversely of the oven band, and a plurality of spaced-apart slots in the feeder waveguide, the slots facing longitudinally of the oven for emitting radiation into the oven generally in a longitudinal direction of the oven with a plane of polarisation substantially perpendicular to the oven band at least a portion of the launch means and the band defining therebetween a microwave treatment through which the products are conveyed in use for being subjected to microwave radiation, the launch means being configured to maintain the polarisation of the radiation in the treatment space substantially perpendicular to the oven band, characterised in that the microwave launch means comprises, as viewed in longitudinal vertical section of the oven, a first stage positioned adjacent to the feeder waveguide and a second stage remote from the feeder waveguide, the first stage comprising upper and lower first stage plates disposed above and below respectively the slots and extending generally longitudinally of the oven from the feeder waveguide, the second stage comprising a panel extending closer to the band in proceeding away from the first stage, the upper first stage plate meeting the second stage panel at a junction therebetween, and the upper and lower first stage plates as viewed in longitudinal vertical section of the oven, being essentially mirror images of each other about a longitudinal plane that extends symmetrically through the plurality of slots and parallel to the band. Preferably said upper and lower first stage plates are substantially flat plates, said upper first stage plate extending upwardly and longitudinally, with respect to the band, from said feeder waveguide, said lower first stage plate extending downwardly and longitudinally, with respect to the band, from said feeder waveguide, so as to define a cavity in the near field of the slots which expands in the direction proceeding away from said slots. A parallel plate portion of the applicator may be provided, said parallel plate portion extending substantially parallel to the band and away from the launch section of the applicator, with which the parallel plate portion is continuous. Such a parallel plate portion effectively extends the length of the applicator in the longitudinal direction of the band, to retain the polarisation of the waveform when the products to be heated are low loss products which do not readily absorb the waveform and accordingly allow the waveform to travel further along the oven. According to a second aspect of the invention we provide a microwave tunnel oven for subjecting products conveyed through the oven to microwave radiation comprising a tunnel oven casing, the oven casing comprising spaced apart tunnel oven side-walls and an oven roof connecting the side-walls, an electrically conductive product- supporting conveyor band positioned between the side-walls and beneath the roof to extend lengthwise of the tunnel, band drive means operative to drive the band, a microwave generator means, a microwave applicator positioned between the side-walls and above the band, microwave supply means connecting the generator means to the applicator, the applicator comprising an elongate microwave emitter assembly extending transversely of the oven band, microwave launch means extending longitudinally of the oven and above the band from adjacent to the emitter assembly, the emitter assembly comprising a plurality of spaced-apart microwave emitter means for emitting radiation Into the oven generally 1n a longitudinal direction of the oven with a plane of polarisation substantially perpendicular to the oven band, at least a portion of the launch means and the band defining therebetween a microwave treatment space through which the products are conveyed in use for being subjected to microwave radiation, the launch means being configured to maintain the polarisation of the radiation in the treatment space substantially perpendicular to the oven band characterised in that the arrangement is such that the mode of radiation in the treatment space is at least predominantly the TM, mode. According to a third aspect of the invention we provide a microwave tunnel oven for subjecting products conveyed through the oven to microwave radiation comprising a tunnel oven casing, the oven casing comprising spaced apart tunnel oven side-walls and an oven roof connecting the side-walls, an electrically conductive product- supporting conveyor band positioned between the side-walls and beneath the roof to extend lengthwise of the tunnel, band drive means operative to drive the band, a microwave generator means, a microwave applicator positioned between the side-walls and above the band, microwave supply means connecting the generator means to the applicator, the applicator comprising an elongate microwave emitter assembly extending transversely of the oven band, microwave launch means extending longitudinally of the oven and above the band from adjacent to the emitter assembly, the emitter assembly comprising a feeder waveguide extending transversely of the oven band and a plurality of spaced-apart microwave emitter means fed by the feeder waveguide, for emitting radiation into the oven generally in a longitudinal direction of the oven with a plane of polarisation substantially perpendicular to the oven band, at least a portion of the launch means and the band defining therebetween a microwave treatment space through which the products are conveyed in use for being subjected to microwave radiation, the launch means being configured to maintain the polarisation of the radiation in the treatment space substantially perpendicular to the oven band, characterised in that said plurality of microwave emitter means comprises a plurality of antennas depending downwardly from said feeder waveguide, and a plurality of probes located in said waveguide, each said probe being directly connected to a respective one of the antennas. According to a fourth aspect of the invention we provide a microwave tunnel oven for subjecting products conveyed through the oven to microwave radiation comprising a tunnel oven casing, the oven casing comprising spaced apart tunnel oven side-walls and an oven roof connecting the side-walls, an electrically conductive product- supporting conveyor band positioned between the side-walls and beneath the roof to extend lengthwise of the tunnel, band drive means operative to drive the band, a microwave generator means, a microwave applicator positioned between the side-walls and above the band, microwave supply means connecting the generator means to the applicator, the applicator comprising an elongate microwave emitter assembly extending transversely of the oven band, microwave launch means extending longitudinally of the oven and above, the band from adjacent to the emitter assembly, the emitter assembly comprising a feeder waveguide extending transversely of the oven band, and a plurality of spaced-apart microwave emitter means fed by the feeder waveguide for emitting radiation into the oven generally in a longitudinal direction of the oven with a plane of polarisation substantially perpendicular to the oven band, at least a portion of the launch means and the band defining therebetween a microwave treatment space through which the products are conveyed in use for being subjected to microwave radiation, the launch means being configured to maintain the polarisation of the radiation in the treatment space substantially perpendicular to the oven band, characterised in that the plurality of microwave emitter means are arranged as a phased array, such that there 1s a uniform phase difference between adjacent pairs of emitter means of said plurality of emitter means, and that the feeder waveguide is located externally of the oven casing, and the plurality of microwave emitter means is connected with the feeder waveguide by a plurality of coaxial waveguide links, the links extending through the oven casing. According to a fifth aspect of the invention we provide a microwave tunnel oven for subjecting products conveyed through the oven to microwave radiation comprising a tunnel oven casing the oven casing comprising spaced apart tunnel oven side-walls and an oven roof connecting the side-walls, an electrically conductive product- supporting conveyor band positioned between the side-walls and beneath the roof to extend lengthwise of the tunnel, band drive means operative to drive the band, a microwave generator means, a microwave applicator, positioned between the side-walls and above the band, microwave supply means connecting the generator means to the applicator, the applicator comprising an elongate microwave emitter assembly extending transversely of the oven band, microwave launch means extending longitudinally of the oven and above tne band from adjacent to the emitter assembly, the emitter assembly comprising a plurality of spaced-apart microwave emitter means for emitting radiation into the oven generally in a longitudinal direction of the oven with a plane of polarisation substantially perpendicular to the oven band, at least a portion of the launch means and the band defining therebetween a microwave treatment space through which the products are conveyed in use for being subjected to microwave radiation, the launch means being configured to maintain the polarisation of the radiation in the treatment space substantially perpendicular to the oven band, characterised by a microwave containment assembly extending for substantially the full length of the applicator, in the longitudinal direction of the band, and comprising an electrically conductive floor extending beneath the band, and spaced-apart electrically conductive containment side-walls on either side of the band, and connecting the conductive floor with the applicator. A sixth spect of the invention comprises a method of heating an article by conveying the article through an oven in accordance with any one of the five previous aspects. A seventh aspect of the invention comprises an article that has been heated by said method. Various embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic longitudinal vertical cross-section of one of the microwave zones of a first combined microwave-biscuit oven, in accordance with the invention, showing an applicator in transverse cross-section, the applicator comprising transverse waveguides incorporating slots, in combination with associated launch sections, Figure 2 is a section in the line A-A of Figure 1 and looking in the direction of travel of the oven band, to show the broad, slotted side of one of the pair of slotted waveguides, Figure 3 1s a schematic longitudinal vertical cross-section of the microwave zone of a second combined microwave-biscuit oven in accordance with the invention, using transverse waveguides incorporating probe-fed monopole aerials in combination with associated launch sections, Figure 4 is a section on the line B-B of Figure 3 and looking in the direction of travel of the oven band, of a waveguide incorporating probe-fed monopole aerials, Figure 5 shows a modification of the oven of Figures 3 and 4 in which the waveguide fitted with probes is external to the oven housing, Figure 6 is a section similar to Figure 1 of a modified combined microwave biscuit oven, in accordance with the invention, and which utilises the TM- j mode as well as the TEM mode, Figure 7 is a section similar to Figure 3 but of a modification to the oven of Figure 6, Figure 8 is a side elevation of a parallel plate transmission line supporting the TM- | mode and showing the electric field lines 1n the volume between the parallel plates, and Figure 9 is a schematic vertical cross-section, similar to Figures 2 or 4, of a modification in accordance with the invention. With reference to Figure 1, an oven comprises side-walls S and a roof R. The side walls S are spaced-apart by a distance of typically 1 or 1.2 metres. Food products 1 are conveyed on a horizontal steel oven band 2 extending between side walls S through a vertically polarised E-field shown E and E. , broadcast from an array of slots 8 formed on oppositely facing broad sides 3 ' of a pair of oblong-rectangular cross-section feeder waveguides 3 of a double applicator. Launch sections 5 of the applicator consist of first launch stage 6, adjacent to the respective waveguide 3, and a second launch stage remote from the respective feeder waveguide 3 1n the form of a panel 7, curved at a radius R, to connect the end of the first launch stage smoothly with horizontal plates 11 of the applicator. A respective treatment space 7' is defined between the panel 7 and the band 2. Thus the first stage 6 of each launch section 5 is defined by an upper first stage plate 6' and a respective lower first stage plate 6". The upper first stage plate 6' in proceeding from the associated waveguide 3 1s directed upwardly and longitudinally of the oven, at an angle of 0 to the plane of band 2, whereas the respective lower first stage plate 6" lies beneath the upper first stage plate 6' and extends downwardly in proceeding longitudinally of the oven from the waveguide 3, from below the slots 8, at the same angle 0 relative to the plane of the band 2. Thus the upper and lower first stage plate pairs define between them a respective cavity 15, in the near field of the slots 8, which expands 1n the direction proceeding away from the slots of the respective feeder waveguide 3. The longer second stage panel 7 extends closer to the band 2 in proceeding longitudinally of the oven from the connection between the second stage panel 7 and the extremity of the respective upper first stage plate 6' to merge with the horizontal plates 11 of the applicator. The applicator, as shown in Figure 2, extends for almost the full width of the oven, and the cross-sectional configuration shown in Figure 1 is uniform for that full length of the applicator. As shown in Figure 2 the feeder waveguides extend through one wall S of the oven for connection to a remote microwave generator means. With reference to Figure 2, food products 1 are conveyed on oven band 2 under the applicator 3. Slots 8 of length d, are formed in one broad side 3' of each waveguide 3 pitched evenly by dimension d and arranged evenly and alternately about the centre-line 4 of the side 3' at a distance d 3 . This may require a flare to compensate for a bias of power transmitted from the first to the last slot. (Microwave theory would Imply that the spacing about the centre-Hne of each slot should be different as the effect of neighbouring slots varies from slot to slot, depending upon their relative positions. (These variations are small in practice and can be ignored.) The positions of the edges 9 of the first stage plates 6 ' , 6" of the launch section 5 are arranged so that the centre-Hne of the slots 1s equidistant between the edges 9 and the centre-Hne 4 of the broad face 3' of the respective waveguide. A base plate 13 extends from the lower edges of lower panels 6", and also defines the lower short side of the feeder waveguides 3, the base plate 13 extending parallel to the band 2 to define between the plate 13 and band 2 a further treatment space 14. Standing waves created in space 14 . by reflections from the launch sections 7 provide additional heating of the products 1 as they pass beneath plate 13. Further treatment spaces IT are defined between the horizontal plates 11 and band 2. Depending on the nature of the products being heated most of the microwave energy will have been absorbed by the products in the treatments space 7 ' and little radiation will reach space IT. However, for some products heating will take place in space IT. In the embodiment of Figure 1 the upper and lower first stage plates 6' and 6" are set at equal angles 0 relative to the plane of the band 2. In modifications, not illustrated, the plates 6' and 6" could be of different shape and orientation but best results are likely to be achieved when the plates 6' and 6" are arranged in mirror image configuration relative to that plane which includes the centre-line of the slots 8 and is parallel to the plane of the band 2. In Figures 3, 4 and 5 parts corresponding to those of Figures 1 and 2 have been given corresponding reference numerals. Referring now to Figures 3 and 4, food products 1 are conveyed on a steel oven band 2 beneath an applicator which creates a vertically polarised E-field, shown E and E. , broadcast from an array of probe-fed monopole antennas 10 located on the bottom face of two rectangular cross-section spaced-apart feeder waveguides 3. Each antenna 10 is fed by a respective probe 10'. The launch sections 5 1n this case each consist of a flared panel structure 7 set at a radius R« to extend between the waveguide 3 and portions 11 of the applicator parallel to the band 2. Dimension ά-. is determined by the power requirements. In the embodiment of Figures 1 and 2, the applicator uses a pair of waveguides 3 extending substantially transverse to the direction of travel of the steel band conveyor 2, each of the waveguide feeds being of oblong-rectangular cross-section, as shown in Figure 1, with the longer dimension of this cross-section disposed vertically and with the slots 8 formed on one of the broad sides 3 ' . This arrangement is used in combination with the launch section 5 to direct the waveform 'through' the top plate of the equivalent parallel plate waveguide. The length of the slots 8 1s substantially one half the free-space wavelength for the chosen frequency and the slots 8 are spaced apart by one half guide-wavelength. Adjacent slots 8 are arranged alternately on opposite sides of the centre-Hne 4 of the broad face 3' of the waveguide 3 to allow for phase reversal of the waveform in the guide. In this way a substantially uniform microwave illumination can be achieved. Additionally, by arranging a second similar pattern of slots 8 to be repeated on the outwardly-fadng broad face 3' of the second waveguide 3, but out-of-phase, in the transverse direction of the band, to the first set of slots by an amount equal to one quarter guide-wavelength, compensation for any non-uniformity of microwave illumination can be achieved, in that the accumulated exposure of a single product to the waveform will be evened out when the product has passed through the two fields generated by the respective waveguides 3. The launch section 5 of the applicator of Figures 1 and 2 is arranged initially to ensure symmetry 1n the very near field of the slots 8 and then to provide a guide for the waves 'through' the top plate of the equivalent parallel plate waveguide. The exact dimensions are determined empirically to achieve a substantially uniform microwave Illumination across the oven band and to maintain vertical polarisation of the E-field. In the arrangement of Figures 3 and 4, the applicator uses an array of probe-fed monopole antennas distributed across the width of the oven but located on the underside of the transverse feeder waveguide. This arrangement is used in combination with a launch section 5 arranged to direct the waveform ' through' the top plate of the equivalent parallel plate waveguide. The length of each of the monopoles 10 is preferably substantially one quarter the wavelength of the free-space waveform for the chosen frequency. Folded monopoles, as shown in Figure 4, are preferred because they are self-supporting and do not require a ceramic holder. The power broadcast from each antenna 10 is proportional to the protrusion of the associated probe 10' into the waveguide, this being arranged to suit the power transmission requirements. The configuration of the launch sections 5 in Figure 3 is based on similar principles to those used for the slotted waveguide of Figures 1, 2 but in this case the provision of plates such as plates 6', 6' used in Figures 1, 2 to ensure symmetry in the near field is unnecessary. Thus, the curved plate 7 extends directly from the respective feeder waveguide 3. Additionally, and in a similar manner to the slotted configuration of Figures 1, 2, the arrangement of probes 10 is repeated on a second waveguide 3 but so as to be out-of-phase with respect to the antennas on the first waveguide 3, in the transverse direction of the band, by an amount equal to one quarter the guide wavelength; in a like manner this compensates overall for any non-uniformity of microwave illumination. The exact dimensions of the launch sections 5 are determined empirically to achieve a substantially uniform microwave illumination across the oven band and to maintain vertical polarisation of the E-field, but approximate values can be related to the wavelengths used. - 17 The wavelengths referred to are determined by the choice of frequency and can be expressed as follows: A = C/f where A is the free-space wavelength of the waveform f is the chosen frequency in Hertz c is the speed of propagation of the waveform in free-space E„ is the relative dielectric constant and where is the guide-wavelength is the factor related to the waveguide, and is typically twice the broad dimension of waveguide With reference to Figures 1, 2, 3 and 4, the dimensions noted have typical values defined as follows: d ] = A/2 d2 - y 2 d, is symmetrical about the centre-line of the broad face of the waveguide and is found experimentally by means of power reflection measurements to give equal power drop per slot pair. d. should be the minimum possible compatible with the construction of the waveguides. dr = 6λ (minimum) d 6 - λ/4 ά j is proportional to the power transmission requirement dg = 1-3A (approx) dg = 3.3> d, Q = 40mm (this is selected to be a minimum to clear the product) R-, and R~ are in the range of 4 to 5 0 is in the range of 10 to 20 degrees. Figure 5 shows that the feeder waveguide 3 fitted with probes 10' may be positioned external to the oven, the probes 10' being connected to respective antennas 10 positioned within the oven by respective coaxial waveguide links 15. Whereas the ovens of Figures 1 to 5 utilise the TEM mode, those of Figures 6 and 7 principally utilise the TM, mode, but with some TEM mode in the upstream and downstream portions of the applicator. In Figures 6 and 7 corresponding reference numerals have been applied to parts which correspond to the ovens of Figures 1 to 5. With reference to Figure 6, the central portion of the applicator, comprising the pair of waveguides 3 with slots in their broad sides 3', respective upper and lower plates 6 ' and 6", and base plate 13, is essentially the same as the central portion of the applicator of Figure 1, but the upper plates 6' connect with respective horizontal main applicator plates 20 which extend parallel to the metal band 2 at a spacing d- j - j which is greater than 60 mm, typically 70 mm, in order to establish the TM, mode in the respective treatment spaces 21 defined between the band 2 and the main applicator plates 20. Figure 8 shows schematically the distribution of the electric field in the equivalent parallel plate transmission line supporting the TM, mode. Immediately above the band the electric field is, in fact, parallel to the band, but in proceeding away from the band the electric field rapidly becomes normal to the band, so that effectively the field in the product is perpendicular to the band. At the mid-height the electric field lines again become horizontal. Therefore the products would preferably be chosen to have a maximum height less than half d,,, and this represents the effective treatment space in this case. As will be appreciated by the skilled addressee, the TM, mode may alternatively be designated the TM Q , mode, since there are no loops of the electric field in the direction extending parallel to but transversely of the metal band 2. As shown in Figure 6, the ends of the main plates 20 remote from the waveguides 3 are continuous with downwardly and outwardly sloping flat plates 7, extending at an angle of typically 45° to the band, the plates 7 connecting with horizontal plates 11 spaced at a distance d, fi above the band 2. The distance d, Q is typically 15 to 20 mm in order to support the TEM mode in the further treatment spaces IT. Since the products to be heated, biscuits 1 being shown in Figure 6, are supported on the band 2 and therefore occupy the region immediately above the band, the E-field to which the products are exposed in the main treatment spaces 21 extends substantially normal to the plane of band 2. The length of d,„ of the main plates 20 in Figures 6 is typically 2 Figure 7 shows an equivalent arrangement to that of Figure 6 but employing antennas 10 in a similar manner to Figure 3. The dimensions d, 0 , d,, and d-.^ are typically as given for Figure 6. Again, the TM, mode is supported 1n the main treatment spaces 21, whereas the TEM mode is supported in the further treatment spaces IT. With reference to Figure 9, a modification 1s shown which can be applied to any of the embodiments of Figures 1 to 8. In order to prevent escape of the microwaves from the sides of the parallel plate transmission line and to contain all microwaves (within safety limits) inside the treatment space/s, a microwave circuit 22 is defined to enclose the emitter assembly, launch section and oven band. As shown,the oven band 2 is supported by electrically conductive skids 23, running lengthwise of the band 2, on the base 25 of the microwave containment circuit 22 of channel section, the band 2 and the applicator panel 11 forming a parallel plate transmission line as previously described. Chokes are arranged at the opposite ends of the applicator, where the products enter and leave. The microwave circuit 22 is continuous along the length of the treatment zone and the sides 26 of the circuit 22 are connected by means of a respective welded or slideable joint at J to the applicator 1n such a way as to ensure substantially no leakage of microwaves at the join.

Claims: AMENDED CLAIMS [received by the International Bureau on 12 October 1994 (12.10.94) ; original claims 8, 1, 2, 3, 4, 5, 6 and 7 replaced by amended claims 1-8 respectively (6 pages)] 1. A microwave tunnel oven for subjecting products conveyed through the oven to microwave radiation comprising a tunnel oven casing (R, S), the oven casing comprising spaced apart tunnel oven side-walls (S) and an oven roof (R) connecting the side-walls, an electrically conductive product-supporting conveyor band ( 2 ) positioned between the side-walls (S) and beneath the roof ( R) to extend lengthwise of the tunnel, band drive means operative to drive the band, a microwave generator means, a microwave applicator (3, 5, 11) positioned between the side-walls and above the band, microwave supply means connecting the generator means to the applicator, the applicator comprising an elongate microwave emitter assembly (3) extending transversely of the oven band, microwave launch means extending longitudinally of the oven and above the band from adjacent to the emitter assembly, the emitter assembly comprising a plurality of spaced-apart microwave emitter means for emitting radiation into the oven generally in a longitudinal direction of the oven with a plane of polarisation substantially perpendicular to the oven band, at least a portion of the launch means and the band defining therebetween a microwave treatment space through which the products are conveyed in use for being subjected to microwave radiation, the launch means being configured to maintain the polarisation of the radiation in the treatment space substantially perpendicular to the oven band, characterised by a microwave containment assembly (22) extending for substantially the full length of the applicator, in the longitudinal direction of the band, and comprising an electrically conductive floor (25) extending beneath the band ( 2), and spaced-apart electrically conductive containment side-walls (26) on either side of the band, and connecting the conductive floor with the applicator. 2. A microwave tunnel oven for subjecting products conveyed through the oven to microwave radiation comprising a tunnel oven casing (R, S), the oven casing comprising spaced apart tunnel oven side-walls (S) and an oven roof (R) connecting the side-walls, an electrically conductive product-supporting conveyor band (2 ) positioned between the side-walls (S) and beneath the roof ( R ) to extend lengthwise of the tunnel, band drive means operative to drive the band, a microwave generator means, a microwave applicator ( 3, 5, 11) positioned between the side-walls and above the band, microwave supply means connecting the generator means to the applicator, the applicator comprising an elongate microwave emitter assembly (3, 8) extending transversely of the oven band, microwave launch means ( 5 ) extending longitudinally of the oven and above the band ( 2) from adjacent to the emitter assembly, the emitter assembly comprising a feeder waveguide (3) extending transversely of the oven band, and a plurality of spaced-apart slots (8) in the feeder waveguide, the slots (8) facing longitudinally of the oven for emitting radiation into the oven generally in a longitudinal direction of the oven with a plane of polarisation substantially perpendicular to the oven band (2), at least a portion (7) of the launch means and the band (2) defining therebetween a microwave treatment space (7') through which the products (1) are conveyed in use for being subjected to microwave radiation, the launch means being configured to maintain the polarisation of the radiation in the treatment space (7') substantially perpendicular to the oven band, characterised in that the microwave launch means (5) comprises, as viewed in longitudinal vertical section of the oven, a first stage (6', 6") positioned adjacent to the feeder waveguide (3), and a second stage (7) remote from the feeder waveguide, the first stage comprising upper (6') and lower (6") first stage plates disposed above and below respectively the slots ( 8), and extending generally longitudinally of the oven from the feeder waveguide, the second stage comprising a panel (7) extending closer to the band (2) in proceeding away from the first stage, the upper first stage plate (6') meeting the second stage panel (7) at a junction therebetween, and the upper and lower first stage plates (6', 6"), as viewed in longitudinal vertical section of the oven, being essentially mirror images of each other about a longitudinal plane (4) that extends symmetrically through the plurality of slots (8) and parallel to the band ( 2). 3. An oven as claimed in claim 2 characterised by a microwave containment assembly (22) extending for substantially the full length of the applicator, in the longitudinal direction of the band, and comprising an electrically conductive floor ( 25) extending beneath the band (2), and spaced-apart electrically conductive containment side-walls (26) on either side of the band, and connecting (at J) the conductive floor with the applicator. 4. An oven as claimed in claim 2 or claim 3 characterised in that the junction between the upper first stage plate (6') and the second stage plate (7) is an angled junction. 5. A microwave tunnel oven for subjecting products conveyed through the oven to microwave radiation comprising a tunnel oven casing (R, S), the oven casing comprising spaced apart tunnel oven side-walls (S) and an oven roof (R) connecting the side-walls, an electrically conductive product-supporting conveyor band (2) positioned between the side-walls (S) and beneath the roof (R) to extend lengthwise of the tunnel, band drive means operative to drive the band, a microwave generator means, a microwave applicator (3, 5, 11) positioned between the side-walls and above the band, microwave supply means connecting the generator means to the applicator, the applicator comprising an elongate microwave emitter assembly (3) extending transversely of the oven band, microwave launch means (5) extending longitudinally of the oven and above the band from adjacent to the emitter assembly, the emitter assembly comprising a plurality of spaced-apart microwave emitter means (8) for emitting radiation into the oven generally in a longitudinal direction of the oven with a plane of polarisation substantially perpendicular to the oven band, at least a portion of the launch means and the band defining therebetween a microwave treatment space (21) through which the products are conveyed in use for being subjected to microwave radiation, the launch means (5) being configured to maintain the polarisation of the radiation in the treatment space ( 21 ) substantially perpendicular to the oven band (2), characterised in that the arrangement is such that the mode of radiation in the treatment space (21) is at least predominantly the TM, mode. 6. An oven as claimed in claim 5 characterised in that the applicator comprises a plate (11) extending substantially parallel to the band and positioned remote from the emitter assembly ( 3, 8) to define a further treatment space (IT) between the plate (11) and the band (2), the arrangement being such that the mode of radiation in the further treatment space (IT) is at least predominantly the TEM mode. 7. A microwave tunnel oven for subjecting products conveyed through the oven to microwave radiation comprising a tunnel oven casing (R, S), the oven casing comprising spaced apart tunnel oven side-walls (S) and an oven roof (R) connecting the side-walls, an electrically conductive product-supporting conveyor band (2) positioned between the side-walls (S) and beneath the roof (R) to extend lengthwise of the tunnel, band drive means operative to drive the band, a microwave generator means, a microwave applicator (3, 5, 11) positioned between the side-walls and above the band, microwave supply means connecting the generator means to the applicator, the applicator comprising an elongate microwave emitter assembly (3) extending transversely of the oven band, microwave launch means (5) extending longitudinally of the oven and above the band from adjacent to the emitter assembly, the emitter assembly comprising a feeder waveguide ( 3) extending transversely of the oven band (2), and a plurality of spaced-apart microwave emitter means (10) fed by the feeder waveguide for emitting radiation into the oven generally in a longitudinal direction of the oven with a plane of polarisation substantially perpendicular to the oven band, at least a portion (7) of the launch means and the band (2) defining therebetween a microwave treatment space (7') through which the products (1) are conveyed in use for being subjected to microwave radiation, the launch means being configured to maintain the polarisation of the radiation in the treatment space substantially perpendicular to the oven band, characterised in that said plurality of microwave emitter means comprises a plurality of antennas (10) depending downwardly from said feeder waveguide (3), and a plurality of probes (10') located in said waveguide, each said probe (10') being directly connected to a respective one of the antennas ( 10). 8. A microwave tunnel oven for subjecting products conveyed through the oven to microwave radiation comprising a tunnel oven casing (R, S), the oven casing comprising spaced apart tunnel oven side-walls (S) and an oven roof (R) connecting the side-walls, an electrically conductive product-supporting conveyor band (2) positioned between the side-walls (S) and beneath the roof (R) to extend lengthwise of the tunnel, band drive means operative to drive the band, a microwave generator means, a microwave applicator (3, 5, 11) positioned between the side-walls and above the band, microwave supply means connecting the generator means to the applicator, the applicator comprising an elongate microwave emitter assembly (3) extending transversely of the oven band, microwave launch means (5) extending longitudinally of the oven and above the band (2) from adjacent to the emitter assembly, the emitter assembly comprising a feeder waveguide (3) extending transversely of the oven band, and a plurality of spaced-apart microwave emitter means fed by the feeder waveguide for emitting radiation into the oven generally in a longitudinal direction of the oven with a plane of polarisation substantially perpendicular to the oven band, at least a portion of the launch means and the band defining therebetween a microwave treatment space through which the products are conveyed in use for being subjected to microwave radiation, the launch means (5) being configured to maintain the polarisation of the radiation in the treatment space substantially perpendicular to the oven band, characterised in that the plurality of microwave emitter means (10) are arranged as a phased array, such that there is a uniform phase difference between adjacent pairs of emitter means (10) of said plurality of emitter means, and that the feeder waveguide (3) is located externally of the oven casing (R S), and the plurality of microwave emitter means (10) is connected with the feeder waveguide (3) by a plurality of coaxial waveguide links (15), the links (15) extending through the oven casing.

Inventors: DAINES PETER NICHOLAS, MEREDITH ROGER JOHN, SHUTE MARTIN RALPH, DAINES, PETER, NICHOLAS, MEREDITH, ROGER, JOHN, SHUTE, MARTIN, RALPH
Applicants: APV CORP LTD, DAINES PETER NICHOLAS, MEREDITH ROGER JOHN, SHUTE MARTIN RALPH, APV CORPORATION LIMITED, DAINES, PETER, NICHOLAS, MEREDITH, ROGER, JOHN, SHUTE, MARTIN, RALPH
Application year: 1994
Main classification: H05B6
IPC codes: H05B6